Effect of concrete properties on cyclic behavior of poorly engineered RC beam-column wall joint

Post-earthquake observations have shown the reinforced concrete beam-column joints of dated structures may result in premature brittle failure during earthquakes. These critical zones transfer complex combination of forces between beams and columns and their premature failure may cause the catastrophic collapse of structures even in areas from low to moderate seismic risk. External beam-column joints (i.e., "wall" and "corner" joints) of buildings constructed without or with insufficient transverse reinforcement are often involved in this types of collapses. The aim of this study is to investigate how the structural response of RC beam-column joints is affected by several mechanical parameters of the concrete, including: compressive strength, tensile strength, Young's modulus, Poisson ratio and fracture energy in tension of the concrete. Both experimental tests and numerical models have been carried out. In particular, four RC beam-column sub-assemblies with no shear reinforcement in the joint have been designed, constructed and tested un- der cyclic loads. In particular, to reproduce the worst possible condition for the beam-column joints, no axial load was applied to the column the tests were performed with no axial load in the column. Experimental results were then compared with those predicted through numerical simulation carried out with a 3D FEM model, able to describe crack propagation in concrete. Experimental structural ductility of the tested RC frames were found surprisingly high and force-displacement numerical curves were found to match experimental ones both in terms of initial stiffness and maximum strength capacity. Sensitivities analysis carried out with respect of various concrete properties showed: (i) the frame strength capacity and ductility to increase with fc; (ii) in case of low concrete strength classes, the collapse of RC frames is expected to be in shear mode; (iii) for high concrete strength classes the collapse of RC frames may be reached in the beam column joint in pressure-bending mode; (iiii) both the maximum strength capacity and the ductility of the RC frames increase as the concrete fracture energy increases; (iiiii) Poisson ratio and concrete tensile strength seem not to have much influence on the mechanical behavior of the RC frame.